How does OR work in an IF statement?

I want the IF statement to check the value of 'Highest_value_month' and give 'Days' a value based on that, yet 'Days' always gets the value 31. As far as I know the syntax for OR is '||'. So why doesn't my IF statement work?

Highest_value_month = 6;

 if Highest_value_month == 1 || 3 || 5 || 7 || 8 || 10 || 12
Days = 31;
elseif Highest_value_month == 2
Days = 28;
else
Days = 30;
end

The code is simplified in this case but it is essentially the same.

Answer 1

[Rik]

Because each is treated as separate test. Therefor you will need to use something like the line below. You could also use switch and case (see second example).

if Highest_value_month == 1 || ...
        Highest_value_month == 3 || ...
        Highest_value_month == 5 || ...
        Highest_value_month == 7 || ...
        Highest_value_month == 8 || ...
        Highest_value_month == 10 || ...
        Highest_value_month == 12
    Days = 31;
end

or with switch:

switch Highest_value_month 
    case {1,3,5,7,8,10,12}
        Days = 31;
    case 2
        Days = 28;
    otherwise
        Days = 30;
end

Answer 2

[KSSV]

Highest_value_month = 6;
val = [1 3 5 7 8 10 12] ;
if intersect(Highest_value_month,val)
    Days = 31;
elseif Highest_value_month == 2
    Days = 28;
else
    Days = 30;
end

OR

Highest_value_month = 12;
val = [1 3 5 7 8 10 12] ;
if ismember(Highest_value_month,val)
    Days = 31;
elseif Highest_value_month == 2
    Days = 28;
else
    Days = 30;
end